1. Field
The following description relates to a PWM controlling apparatus, such as for a flyback converter. The following description also relates to a device that controls a flyback converter with an averaging/sampling method of an auxiliary winding voltage or an average current mode method.
2. Description of Related Art
Recently, use of portable electronic devices such as laptops, mobile phones, tablet PCs, and so on is increasing. The size of the portable electronic devices is tending to decrease, for improved portability. However, the functionality of the battery used to power the portable electronics is not improving as such scaling down occurs. Hence, the functionality of portable electronic devices is encountering issues and carrying an external power device is becoming required to supply power to such portable electronic devices because of increasing energy consumption requirements.
An adaptor using an isolated DC-DC converter, for example, a flyback converter, is mainly used as a power supply device of a portable electric device. A flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. For example, an application such as an adapter is controlled to have a constant current (CC) mode method and a constant voltage (CV) mode method. Further, a feedback signal is possibly received from a secondary-side to control the CC/CV control. The feedback signal generally uses an opto-coupler. For example, an opto-coupler uses light to transfer signals between otherwise isolated circuits. A primary side receives the secondary side information of the flyback converter through the opto-coupler and the controller controls an output voltage or an output current accordingly.
Likewise, the flyback converter receives a feedback signal from a secondary side that is an output terminal of the flyback converter for the CC/CV control operation. However, this structure has an issue of using a complicated secondary side circuit. In other words, in the example of the flyback converter, the primary side and the secondary side are insulated from one another by a transformer. Accordingly, circuits such as a secondary side circuit and an opto-coupler are required as aforementioned. Thus, the circuit structure accordingly becomes more complicated.
The primary side control method, referred to as Primary-Side Regulation (PSR), that controls the flyback converter to improve this situation has been suggested. The method controls a secondary side voltage or a current by indirectly obtaining the secondary information from the auxiliary winding voltage. Accordingly, a feedback circuit is not required to be formed, and thereby a lower cost, a smaller area and a higher efficiency become possible. However, because there is a requirement for secondary information to be transferred indirectly, a regulation feature in this method is further degraded than a regulation feature in the method that uses the opto-coupler. Further, in this approach, an issue of being influenced by a secondary side diode voltage drop is generated during constant voltage control. Therefore, in an application requiring an accurate regulation feature, a method controlled by forming a feedback circuit using an opto-coupler from the secondary-side is used.
However, recently, although the regulation feature is somewhat degraded, a primary side control method is preferred to obtain the advantages in terms of efficiency, area and cost.
As an example of the primary side control method, an output current is controlled using a peak current mode pulse-width modulation (PWM) method. However, in this example, operation failures may be generated or there may be limits to accurate control of operation due to an effect according to switching noise from a flyback converter.
Furthermore, an error exists between an auxiliary winding voltage and the output voltage due to a forward bias voltage of a diode when a current is applied to a secondary side diode. Further, when the primary switch is off, the current applied to the secondary side diode starts to decrease from a peak and becomes a zero current. The auxiliary winding voltage in this example generates a resonance phenomenon through an inductance of a primary side transformer and series parasitic inductances. Likewise, when a resonance phenomena is generated in the auxiliary winding voltage, the output voltage is not able to be controlled accurately. In other words, an output voltage of the flyback converter is proportional to the auxiliary winding voltage. Thus, a design of a timing and method of accurately sampling the auxiliary winding voltage is highly relevant to accurately control an output voltage.